1. Field of the Invention
To obtain desired responses from microwave systems, microwave filters are generally required. One such response from a microwave system is its capability to transmit only a selected, and often narrow, band of frequencies in the microwave region of electromagnetic energy. This response is generally called a bandpass response. And, the microwave filter used to obtain such a response is generally called a bandpass filter, which is the subject of the present invention.
2. Description of the Prior Art
Microwave bandpass filters are generally constructed from physical cavities of rectangular, cylindrical, or spherical shapes. Filters consisting of a single cavity, or a plurality of linked rectangular or cylindrical cavities are common in the prior art. They are, however, deficient in several aspects.
In single cavity filters, an example of which is shown and described in T. Ishii, "Microwave Engineering," The Ronald Press Co., New York, 1966, pages 82-95, the bandpass responses produced are generally not adequate because the electromagnetic energy outside the desired band is not sufficiently attenuated to provide responses that are generally preferred, such as Tchebychev and elliptic functions.
One approach to provide a microwave filter having the proper bandpass characteristics is to link together cavities. In this instance, the electromagnetic energy, traveling through a chain of identical cavities, is sequentially affected by each cavity's bandpass characteristics. This cumulative effect is generally sufficient to restrict the electromagnetic energy to the preferred responses. This approach, however, causes microwave filters to be bulky and difficult to be adapted for use in size or weight limited environments such as a spacecraft. To alleviate such deficiency, multiple mode filters such as the dual mode cylindrical or rectangular filter disclosed in Blachier et al., U.S. Pat. No. 3,697,898, have been provided.
In dual mode filters, a cavity is provided wherein that cavity is allowed to support two modes of a filter's resonant frequency, a mode being the electric field shape or configuration of that resonant frequency in the cavity. In general, to produce the desired response from a filter, a cavity is configured to allow the passage of only a particular mode of the resonant frequency. The electromagnetic energy, restricted to this mode, emerges from the filter with the desired response. In the instance of the dual mode filter, rather than have only one mode oscillating in a cavity, that one mode is tuned or perturbed to create a second mode. However, the second mode has one difference, that is, the direction of its electric field is orthogonal to the electric field of the first mode. This phenomenon, generally referred to as dual mode, allows the electromagnetic energy to be affected by the cavity's bandpass characteristics twice in one cavity rather than only once. Thus, with two modes rather than one oscillating at the same resonant frequency in a cavity, the number of cavities necessary to produce the desired responses is correspondingly reduced by one-half. The perturbation of the electric field of one mode to produce an orthogonal mode is generally called coupling, which invariably is caused by structural discontinuities in the cavity such as screws positioned on its wall that perturb the electric field of the first mode.
To produce the response characteristics of a multiple cavity filter, these dual mode cavities also may be linked together. The linking mechanism in this instance is generally provided by an aperture between any two cavities. This phenomenon, also generally called coupling, allows the transfer of energy from one mode in the first cavity to another mode in the second in order to have the energy further filtered. In particular, coupling of one mode to a non-adjacent mode must be used. Since the modes are sequentially coupled in a multiple cavity filter, a non-adjacent mode refers to any mode that is not sequentially adjacent to the mode of interest. Filters employing this technique, generally called bridge coupling, are disclosed in Blachier et al., supra, and Atia et al., U.S. Pat. No. 4,060,779.
Since the definition of one mode resonating in a cavity is generally referred to as an electrical section, a dual mode cavity is defined as supporting two sections in each cavity. Accordingly, a three cavity dual mode filter would produce the response characteristics of a six section single mode filter.
Another approach to provide a microwave filter having the proper bandpass characteristics is to maximize the number of modes in a single cavity. One such example is the tri-mode single sphere filter disclosed in Currie, U.S. Pat. No. 2,890,421. In this filter, three non-identical orthogonal modes of the electromagnetic energy reside in a sphere. These modes are coupled together by screws to provide the characteristics of a three-section filter. An alternative embodiment, due to the use of more coupling screws, acts as a five-section filter. This filter, however, lacks the capability to provide the characteristics of a six-section or more filter and, more importantly, the characteristics of a filter having an elliptic function response. This incapacity is due to the presence of uncontrollable and undesirable modes when the original tri-modes are coupled to act as a filter having greater than five sections.